The present invention relates to a clamping device for positioning workpieces. Such devices are used in vises for example to position a workpiece precisely before its procession by drilling, milling etc. The requirements concerning procession have been increasing and demand a more precise positioning. On the other hand the vise should be easy to handle and the positioning should not take a long set-up time.
Positioning devices are known from the prior art. EP 0 761 382 A1 discloses a jaw of a vise for example, which provides a plurality of bores in several horizontal rows in its main body. Positioning pins are slidably located in the bores and can be moved between an active position, in which they project out of the bores, and an inactive position, in which they do not project out of the bores. The workpiece rests at the pushed out pins when clamping between the jaws. The pins can be moved from the inactive position into their active position row by row using pressurized air or another fluid. It is injected via pressurization channels in the jaw into fluid chambers and pushes the pins into their active position. Depending on the shape of the workpiece not all pins are required for support. Some can be pushed back manually pin by pin into the main body of the jaw. Any undesired movement of positioning pins in the bores is prevented by friction, which is strong enough to keep the pins in the active or inactive position, but can be overcome easily by the force applied to the pin manually or by pressurized fluid in the fluid chamber.
This results in both a short set-up time for adapting the vise to a new workpiece and a precise positioning because of the known arrangement of the pins. A plurality of pins is needed, which are rather expensive because of the following requirements:
By pressurizing the air supply the pins must not be pushed out of the jaw, but be held in a defined active position. So they provide a head, which cannot pass through the bore because it is larger than the diameter of the bore. The pins known from the prior art are of piston shape providing two coaxial cylindric sections. When the end surface of the larger section is exposed to pressurized air a part of the smaller section will be projected out of the bore and be stopped in the active position.
The precision of positioning of a workpiece depends on the precision of the radial location of the pins which carry the workpiece before it is clamped in the clamping device. So small work tolerances of pins and bores are required which increase cost.
A pin in the active position can be damaged by a radial shock from the workpiece, for example if it is not put into the clamping device carefully. The hit pin may be bent and stuck in the bore, because after the shock it cannot be removed out of the bore neither to the front (because of its head located in the fluid chamber) nor to the back (because of its bent section being projecting out of the bore), so the whole jaw could become useless. In the prior art the pins provide a predetermined breaking point. If a pin which is in its active position is hit strongly its section which is projected out of the bore is broken off. The remnant of the pin can be moved to the back out of the bore and can be replaced by a new pin easily after removing the rear covering of the jaw.
The required piston shape of the pins and the predetermined breaking point in the smaller section result in a complicated design and rather high production cost, so they are the object of improvements.
The aim of the present invention is to overcome the drawbacks of the prior art by providing pins which fulfill the requirements and cause less production cost because of an easier construction. This object is attained by a positioning device according to claim 1 or a positioning pin according to claim 2.
No predetermined breaking point is necessary as is the case in the prior art for easy removal of damaged pins from the back side. In the present invention the predetermined breaking point is dispensable, because a bent pin can be removed to the front instead of the back side of the jaw. The piston shaped pin does only have one cylindric section with no different diameters which is rather inexpensive. The positioning pin of the present invention comprises a pin body which does not exceed an imaginary enveloping cylindric surface being the axial prolongation of the bore in which it is located, for example a cylinder. The pin body is connected with a stop member which does exceed the imaginary enveloping cylindric surface being the axial prolongation of the bore. In normal operation the pin cannot completely pass through the bore because of the stop member. If necessary a damaged pin can be removed by deforming or destroying the stop member in the working chamber by applying a force directed to the front to the pin body which is greater than the force applied by pressurized fluid in the working chamber. Three preferred embodiments are described below.